The present invention relates to a rolling bearing having a grease composition sealed therein. More particularly, the present invention relates to a rolling bearing which is to be used under high-temperature, high-speed, and heavy-load conditions and is suitable for use in an area susceptible to intrusion of water; for example, automobile electrical equipment, an alternator or intermediate pulley which is auxiliary equipment of an engine, an electromagnetic clutch for an automobile air conditioner, a water pump, an electromagnetic clutch for a gas heat pump, or a compressor.
Intrusion of water into a lubricant usually results in a great drop in the durability and life of a rolling bearing. For instance, it is reported that intrusion of water into lubricating oil (Turbine oil #180) in an amount of 6% results in a drop in rolling fatigue strength to as low as one-twentieth that achieved when nothing is mixed in a lubricating oil [see xe2x80x9cAbout Rolling Fatigue Arising in Surface Starting Point and in Internal Starting Pointxe2x80x9d by Kyozo FURUMURA, Shiniichi SHIROTA, and Kiyoshi HIRAKAWA, NSK Bearing Journal, No. 636, pp. 1 through 10, 1977]. Schatzber et al. have also reported that intrusion of water into lubricating oil in an amount as low as 100 ppm results in a drop in rolling strength of steel as high as 32 to 48% (P. Schatzberg. I. M. Felsen, xe2x80x9cEffects of Water and Oxygen During Rolling Contact Lubrication on Wear,xe2x80x9d No. 12, pp. 331 through 342, 1968).
In many cases, each of the above-described component bearings is used at high temperature and high speed as well as under heavy load. As described in Japanese Patent Publication No. 2878749, oxygen sometimes develops as a result of decomposition of grease. The bearing is employed as an auxiliary belt-drive bearing disposed outside an engine. Hence, the bearing is susceptible to intrusion of muddy water or rainwater splashed from a road surface. A water-pump bearing is also susceptible to intrusion of circulating water for cooling the engine. These bearings are usually constructed so as to prevent intrusion of water from the outside, by means of a contact rubber seal. However, complete prevention of intrusion of water has not yet been achieved. The automobile engine is a machine which is to be repeatedly activated and deactivated. When the bearing is in an inoperative state, the internal temperature of a housing of the bearing drops. When the temperature has reached the dew point, moisture content in air surrounding the bearing condenses to water droplets. The water droplets often adhere to the bearing or intrude into a lubricant. As described in Japanese Patent Unexamined Publication No.Hei 11-72120, hydrogen originating from the thus-intruded water enters into steel of the bearing, thereby inducing exfoliation while involving occurrence of white-etching-constituent structural changes stemming from hydrogen embrittlement.
Each of the component bearings is rotated by a belt and pulley. Static electricity develops between the belt and the pulley. Inner and outer rings are usually insulated from each other by means of an oil film of lubricant during the course of rotation of a bearing. However, when metal contact has arisen for reasons of strong vibration, electrical continuity is established momentarily, thereby inducing occurrence of a great potential difference between the inner and outer rings. The resultant D.C. voltage induces electrolysis of water, thereby accelerating generation of hydrogen ions. As a result, a bearing becomes susceptible to exfoliation while involving occurrence of white-etching-constituent structural changes such as those mentioned above.
As mentioned above, each of the component bearings is susceptible to exfoliation while involving occurrence of white-etching-constituent structural changes due to hydrogen embrittlement stemming from intrusion of water from the outside. Further, exfoliation is accelerated by static electricity. Prevention of acceleration of exfoliation has posed a new important challenge.
Accordingly, it is an object of the present invention to provide a rolling bearing with life which is used at high temperature and high speed and under heavy load and prevents exfoliation, which would otherwise be caused while involving white-etching-constituent structural changes due to hydrogen embrittlement when water has entered the bearing from the outside.
The object, a rolling bearing according to the present invention comprising an inner ring, an outer ring, a plurality of rolling elements rotatably retained between the inner and outer rings and a grease composition which contains a conductive substance in the range from 0.1 wt % to 10 wt % and is sealed in a bearing space defined by the inner and outer rings and the rolling elements.
In particular, in the above-mentioned structure it is advantageous that the plurality of rolling elements are rotatably retained at substantially uniform intervals between the inner and outer rings via the retainer.
In particular, in the above-mentioned structure it is advantageous that the grease composition contains a conductive substance in the range from 0.5 wt % to 5 wt %.
In particular, in the above-mentioned structure it is advantageous that the total amount of additives is usually determined so as to become 20 wt % or less of the overall amount of the grease composition.
In particular, in the above-mentioned structure it is advantageous that the conductive substance comprises carbon black.
In particular, in the above-mentioned structure it is advantageous that a mean particle size of the carbon black ranges from 10 to 300 nm.
In particular, in the above-mentioned structure it is advantageous that the conductive substance comprises carbon nanotube.
In particular, in the above-mentioned structure it is advantageous that the carbon nanotube has a diameter of 0.5 to 15 nm and an overall length of 0.5 to 50 xcexcm.
As mentioned above, generation of hydrogen ions is accelerated by means of electrolysis of water induced by static electricity developing between a belt and a pulley. However, the thus-developing static electricity is energized at all times by means of a conductive substance contained in grease. Hence, a potential difference developing between the inner and outer rings is substantially obviated, thereby preventing occurrence of electrolysis of water. This in turn suppresses generation of and progress in exfoliation involving occurrence of white-etching-constituent structural changes.